Add per-connection master address override for outstation clients

[jadamcrain]

Summary

• Add ConnectionInfo::set_master_address() to allow outstations to use different master addresses
  per connection
• Master address changes persist across reconnections until explicitly changed
• Useful for failover scenarios where an outstation connects to different masters at different
  endpoints

Changes

• Core API: Add set_master_address() to ConnectionInfo builder
  (dnp3/src/tcp/connector.rs:102-112)
• FFI Bindings: Refactor connection handler to use ConnectionInfo builder pattern with master
  address support
• Examples: Update C++ outstation example to demonstrate round-robin connections with per-endpoint
  master addresses
• Code Quality: Replace manual Default implementations with #[derive(Default)] for decode/attr
  enums
• Misc: Update .NET examples to target .NET 8.0

[gemini-code-assist]

Summary of Changes

Hello @jadamcrain, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant enhancement for DNP3 outstation clients, allowing them to dynamically override the master address on a per-connection basis. This feature is crucial for robust failover mechanisms, enabling outstations to connect to different masters at various endpoints. The changes involve updates to the core API, FFI bindings, and example code to facilitate this new capability, alongside minor code quality improvements and .NET target updates.

Highlights

• Per-Connection Master Address Override: Outstation clients can now specify a different master address for each connection attempt using ConnectionInfo::set_master_address().
• Persistent Master Address Changes: Once set, the master address persists across reconnections until explicitly changed, supporting failover scenarios where an outstation connects to different masters at various endpoints.
• FFI Bindings Refactor: The FFI bindings for connection handling have been updated to utilize a ConnectionInfo builder pattern, providing more granular control over connection parameters for C/C++ clients.
• C++ Example Update: The C++ outstation example now demonstrates round-robin connections with per-endpoint master addresses, showcasing the new functionality.
• Code Quality Improvement: Manual Default implementations for several enums (AttrSet, AppDecodeLevel, TransportDecodeLevel, LinkDecodeLevel, PhysDecodeLevel) have been replaced with #[derive(Default)] and #[default] attributes for cleaner and more concise code.
• .NET Example Target Update: The .NET examples have been updated to target .NET 8.0.

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[gemini-code-assist]

Code Review

This pull request introduces a valuable feature for outstation clients, allowing per-connection master address overrides, which is particularly useful for failover scenarios. The implementation is well-structured, propagating the new master_address setting from the ConnectionInfo builder down to the outstation session. The refactoring of the FFI connection handler to a builder pattern is a significant improvement, making the API more extensible and idiomatic, although it introduces breaking changes for FFI consumers. The accompanying updates to examples, especially the C++ outstation example, clearly demonstrate the new functionality. The code quality improvements, such as using #[derive(Default)], are also welcome. I have one suggestion to improve the robustness of the session handling.